Project 5's long term goal is to investigate both the basic biology underlying normal cardiac development and function and the physiological consequences of the myosin heavy chain (MyHC) isoform shifts that occur during the development of heart failure. Many of the signaling pathways activated during compensated hypertrophy and decompensated failure result in fetal gene program activation. Consistent with these data, it has recently been shown that substantial amounts of the alpha-myosin heavy chain gene transcript are present in the normal human heart, and that these levels are dramatically down regulated during heart failure. What is the functional significance of the different cardiac proteins that constitute the main component of the pump? Can modulation of the myosin heavy chain isoform content impact significantly upon the heart's ability to maintain normal cardiac output under normal or pathologic conditions? Project 5 addresses these questions by modulating the myosin isoform content of the mouse heart. SPECIFIC AIM 1 will, using cardiac-specific transgenic expression in the mouse, replace the normal alpha-MyHC isoform with the beta1 beta- MyHC protein. Analyses at the molecular, cellular, whole organ and whole animal levels over the lifetime of the transgenic cohorts will provide a comprehensive picture of the consequences of varying myosin isoform content. SPECIFIC AIM 2 will challenge these mice, using both a surgically-induced pressure overload model, and breeding them into a number of genetically defined, cardiac-compromised transgenic lines, in order to define how altered motor content impacts on the progression and severity of cardiac disease. SPECIFIC AIM 3 will test the relevance of the mouse models to the human condition by creating the exact isoform shift that occurs in man in a transgenic rabbit heart. The alpha-MyHC will be linked to the beta-MyHC promoter in order to effect a partial or complete replacement of the normal beta-MyHC in the rabbit heart. The effects at the molecular and cellular levels, as well as hypertrophy and failure. Finally, SPECIFIC AIM 4 will measure both the RNA and protein levels for the alpha- and beta- MyHC's in populations with defined heart disease. These data will establish the normal and abnormal levels of these molecules during various, well-defined stages of human cardiac disease. The levels of the V1 and V3 will be correlated with the mechanical properties of the fibers. Together, these experiments will, for the first time, unambiguously establish the structure-function relationships of the cardiac myosin isoforms in the intact animal and will help establish the consequences of altered isoform content on compensated hypertrophy and its progression to heart failure.